The method and apparatus disclosed in the prior art are aimed at decreasing energy consumption by the recapture of heretofore unused heat from metallurgical processing gases and exploitation of the retained heat to preheat the fuel gases used in combustion processes, resulting in the conservation of energy and fuel. In the metallurgical field, efforts to conserve heat energy have been principally directed toward decreasing amounts necessary for processing using coke gas, natural gas, blast-furnace gas, heating oil, etc. Another purpose sought to be accomplished by such conservation methods is the utilization of the heat of exhaust gases of metallurgical processes to support the production of high temperatures for the extraction of metal. More specifically, in the production of crude iron and steel it is possible and highly desirable to increase the economy of the metallurgical processes by using higher input temperatures of the processing gases.
The energy expenditure in ore reduction is considerable; the blast furnace itself uses about 66% of the total energy of a metallurgical plant for the production of crude iron. Approximately 3000 Nm.sup.3 of air are required for the burning of one ton of coke. In order to conserve fuel, in this example, coke, the present invention contemplates that the air is heated in air heaters to a maximum temperature of about 1300.degree. C. Processing gas escaping from the metallurgical furnace is recaptured and utilized, with the addition of coke gas, for heating the air. The coke gas is burned in the air heater and subsequently flows through refractory stones, in the shape of gratings. Thus, the interior of the air heater is heated to a maximum temperature of about 1550.degree. C. The exhaust gas flowing out of the air heater still, however, possesses a temperature of approximately 250.degree. C. The exhaust gas of the prior art, still having a considerable heat content, nevertheless leaves the installation through a chimney and flows into the open atmosphere. The prior art teaches that, subsequent to the heating period of the grates, the gas burner of the air heater is switched off, and cold air, produced in blast machines under high pressure, is blown through the hot masonry grating of the air heater. The hot stones release a large portion of their heat into the air which is then blown into the blast furnace through the conventional hot air ring line and blast tuyeres. In the conventional set-up, at least two air heaters are at work at any given time, alternating between heating period and air period to thereby improve the blast capacity.
It is known (Iron and Steel Engineer, August 1979, page 14), that conservation in heat energy is achieved through the use of improved heat exchangers. To this end, it has been proposed to use rotating plate/pipe heat exchangers whose thermo-heat transfer efficiency is presumed higher than that of pipe recuperators. Such heat exchangers have already been employed for pre-heating the air for the air heaters of blast furnaces. The savings in heat, through the employment of heated air instead of the customary cold air in the air heater, amounts to about 25,000 kcal/t crude iron. With a heating value of the coke furnace gas of approximately 4000 kcal/Nm3, this conservation in heat leads to approximate savings of 6 Nm.sup.3 of coke gas per ton of crude iron. While this results in heat energy savings, it is still relatively negligible and accordingly needs to be improved. A further disadvantage of prior art methods and apparatus which use the rotating plate/pipe heat exchangers is that they are exposed to a great deal of erosion at their rotating parts and, therefore, must be periodically replaced when they become worn.
The object of the present invention is to increase the thermal economy and efficiency in the recovery and recycling of heat from hot gases, particularly from metallurgical exhaust gases. A method and apparatus to attain the desired economy and efficiency is an object of the invention disclosed herein. Thermal economy and efficiency is achieved by the present invention by conducting the exhaust gases through gaps located between a plurality of heat pipes which are arranged so as to be separated by spaces and closed on both ends. Subsequently, the exhaust gases are released into the open and fresh cold air, passing by the just heated heat pipes receive the heat energy stored in the heat pipes. Alternatively, cold fuel gases introduced into the gaps, heated to below the range of ultimately desired gas-heat temperature, are fed into a combustion and/or metallurgical process. Preheating of the combustion air, necessarily introduced into the combustion process, results in significant economic advantages for the process. These advantages have an effect, for example, inside the blast furnace and the hot-blast cupola furnace and bring about, inter alia, considerable savings in fuel, specifically of metallurgical coke. The method, according to the present invention, works advantageously with a heat transfer efficiency of greater than 75%.
Additionally, it is especially advantageous to apply the method, according to the present invention, to the blast furnace air-heater method, in which at certain times, at periodic predetermined intervals, air, preheated through the heat pipes, is conducted into the combustion chamber, and burned gases, which initially heat the masonry grating, are subsequently conducted between the heat pipes. The preheating of the air for the air heater, i.e., the introduction of hot air instead of cold air into the air heater during the heating-up period, has heretofore been neglected because the technical relationships were not appreciated. Experts were dissuaded in particular from utilizing the exhaust heat of the air heater for the preheating of the combustion air because the temperature level of the air-heater exhaust gases were considered too low. Now, however, the present invention uses the retained heat of the exhaust gases, albeit low, for heating fresh cold air. Furthermore, the relative low efficiency of the heat transfer of gas recuperators prevented the exploitation of exhaust-gas heat on economical grounds.
It is also advantageous to apply the method of the present invention to the regeneration of zeolitic molecular screening substances, such that the air conducted between the heat pipes is heated to a temperature of between about 200.degree. to 300.degree. C. and subsequently conducted through the molecular screening bed to absorb nitrogen, carbon dioxide and water vapors.
Yet another advantageous feature for practicing the described method is proposed. To this end, it is provided that the heat pipes, arranged in the form of rows or gratings, are combined into a heat exchanger and surrounded with a heat-exchanger casing, whereby the longitudinal axes of the heat pipes extend at right angles to the direction of the flow of cold air or, if applicable, to the direction of the flow of the fuel gas or exhaust gas. Additionally, each longitudinal section of the heat pipes can be surrounded by separate chambers stacked one upon the other. If the latter construction is used then the space-saving arrangement of the heated portions of the pipes with respect to the cooled portions of the heat pipes is especially advantageous. It is an advantage of the present apparatus that the heat-releasing or, if applicable, heat-receiving gas flows in a horizontal direction. This results in corresponding flow cross sections without the necessity for a large re-routing section.
Still a further advantage of the present inventive apparatus lies in the arrangement of the heat pipes in the shape of rows or, if applicable, gratings; i.e., they are connected to one another through rod or lamella-shaped supports. In this case, the supports, made from metals or non-metals, can serve also as heat carriers.
The thermodynamic efficiency of the apparatus, according to the present invention, may also be increased by insulating the separate chambers of the heat exchanger with thermal insulation materials.
Exemplary embodiments of the present invention are illustrated in the drawings and described in more detail below. The inventive method, according to the present invention, is also described below and best understood with the aid of the drawings.